1. Technical Field
This invention relates to wireless transceivers and, more particularly, to an arrangement for providing signal isolation in such transceivers.
2. Description of the Prior Art
Wireless transceivers are in wide use today in various forms including cordless and cellular telephones and other similarly configured portable units. These wireless transceivers have evolved from large bulky units, that operate at low frequencies with an external antenna, to small palm sized units, that operate at high frequencies and utilize an internal antenna.
One of the continuing problems in designing high-frequency wireless transceivers is the undesirable interaction in the form of cross-over modulation that occurs between radio frequency (RF) circuits and audio circuits in such transceivers. When located in close proximity to each other, the audio circuits frequently modulate the RF circuits and cause undesirable interference. This interference may appear in the form of a corrupted transmitted RF signal, resulting in a degraded signal-to-noise ratio in a remote receiver unit. Similarly, the audio circuits can demodulate the RF signals, generating undesirable audible tones which are produced in an audio receiver in the wireless transceiver.
One known solution to this problem commonly practiced by engineers is to utilize RF shielding for the RF circuits and therefore suitably contain the RF energy generated by these circuits in shielded compartments. In that unitary wireless transceivers by design have an audio receiver and an audio transmitter disposed generally at opposite ends of the unit because of modality considerations, it is usually necessary to have an audio circuit near the audio receiver for amplifying the audio output signal and an audio circuit near the audio transmitter for similarly amplifying the audio input signal.
A common audio circuit in the wireless transceiver generally contains the audio processing circuitry necessary for proper operation of this transceiver. Since the audio transmitter and audio receiver, and associated amplifying circuitry for each of these components, are separated in the transceiver unit, it is necessary to provide an isolation path around the RF circuits for at least one of the audio signals for connecting to the common audio circuit. By way of example, the audio signal from the audio transmitter and its amplifying circuitry may by coupled to the common audio circuit which may be configured to also contain the audio receiver and its amplifying circuitry. This arrangement still requires a dedicated routing path for an audio signal, however, which without the proper shielding of the RF circuits may nevertheless interact negatively with the RF signals in the transceiver. Although shielding may be employed to minimize the cross-over modulation between the RF circuits and the audio signal, such shielding is difficult and costly to implement. It is therefore desirable to design a wireless transceiver which provides satisfactory operation without the need for shielding of an audio signal which is routed near or across RF circuits.